In the recovery of the crude oil from underground reservoirs, one known method includes the injection of a solvent into the reservoir to displace the crude oil through the reservoir. When solvents are employed that will mix in all proportions with crude oil at the ambient temperature and pressure of the reservoir from which the crude oil is produced, the term "miscible flooding" is applied to the process. The process of miscible flooding can be extremely effective in stripping and displacing oil through the reservoir. Miscible fluids which have been used include light hydrocarbons and mixtures thereof, such as paraffins in the C.sub.2 -C.sub.6 range, and in particular, liquid petroleum gas (LPG). However, miscible flooding with LPG has not become widespread because of the ready market and high value of LPG, making miscible LPG projects uneconomic.
In another process for the recovery of crude oil from underground reservoirs, a crude oil displacing fluid which is not miscible with the crude oil at the ambient temperature and pressure of the reservoir but which will develop miscibility with the crude oil is injected into the reservoir to displace the crude oil contained in the reservoir. The term "miscible flooding" is also applied to this process. This sort of miscible flooding is termed "developed miscibility," or "multiple-contact miscibility," wherein it is thought that the intermediates (C.sub.2 -C.sub.6) of crude oil transfer into the crude oil displacing fluid over a sustained period of exposure, as opposed to "first-contact miscibility," wherein a zone of contiguously miscible fluids will result.
A mixture of crude oil and a crude oil displacing fluid that will develop miscibility with the crude oil have been observed to form three phase systems when maintained at the temperature and pressure of the reservoir from which the crude oil was produced. The three-phase system comprises an upper vapor phase rich in the miscibility-generating solvent, a middle-phase liquid also rich in the miscibility-generating solvent, and an oil-rich liquid lower phase. A solid asphaltene phase which coexists with the vapor and liquid phases has been observed in some cases.
Carbon dioxide, which is relatively inexpensive compared to LPG, has been used as an oil-recovery solvent. Carbon dioxide is miscible with crude oil in certain reservoirs, but usually at a reservoir pressure less than about 2,000 psia at ambient reservoir temperatures. The minimum pressure at which carbon dioxide is miscible with crude oil from a reservoir is determined at the ambient reservoir temperature and is referred to as the minimum miscibility pressure (MMP).
Carbon dioxide can be mixed with hydrocarbons to produce a displacing fluid that develops miscibility with the crude oil being displaced at the ambient temperature and pressure of the reservoir when the pressure of the reservoir to be flooded lies below the pure carbon dioxide minimum miscibility pressure. Processes utilizing these methods are disclosed in U.S. Pat. Nos. 3,811,501 and 3,811,503, both issued to D. Burnett, et al., on May 21, 1974.
The processes described to produce such carbon dioxide mixtures require mixing carbon dioxide with the required hydrocarbon. The hydrocarbon is expensive and, in some cases, unavailable at field locations.
Other pertinent publications include "Multiple Phase Generation During Carbon Dioxide Flooding", R. L. Henry and R. S. Metcalfe, SPE/DOE Symposium on Enhanced Oil Recovery, Apr. 20-23, 1980 (SPE Paper No. 8812), and "Determination and Predictability of Carbon Dioxide Minimum Miscibility Pressures", W. F. Yellig and R. S. Metcalfe, Journal of Petroleum Technology, January, 1980, pages 160-167, and "Effects of Impurities on Minimum Miscibility Pressure and Minimum Enrichment Levels for CO.sub.2 and Rich Gas Displacements", R. S. Metcalfe, SPE Annual Meeting, 1980 (SPE Paper No. 9230). These publications describe the methods of determining minimum miscibility pressure and multiple phase miscibity.